Silicon carbide (SiC) is a type of compound semiconductor which is thermally and chemically stable. It has properties which are advantageous compared to those of silicon (Si) in that it has approximately three times the band gap, approximately ten times the dielectric breakdown voltage, approximately two times the electron saturation speed, and approximately three times the thermal conductivity of Si. On account of these superior properties, it is anticipated that SiC will be capable of use as a material for electronic devices such as power devices which overcomes the physical limits of Si devices and environmentally resistant devices which operate at high temperatures.
In optical devices on the other hand, nitride-type materials (GaN, AlN) are being developed with the object of shortening wavelengths. SiC has a particularly small lattice misfit with respect to the nitride-type materials compared to other compound semiconductors, so it has attracted attention for use as a substrate for epitaxial growth of nitride-type materials.
However, SiC is well known as a material having polytypes (crystalline polymorphism). Polytypes are the state that the same stoichiometric composition has a plurality of crystalline forms which are different with respect to the mode of stack of atoms only in the c-axis direction. Typical polytypes of SiC include 6H type (a hexagonal system having six molecules per period), 4H type (a hexagonal system having four molecules per period), and 3C type (a cubic system having three molecules per period). Coexistence of at least two different crystal types is undesirable from the standpoint of application thereof to devices.
In order to apply SiC to electronic devices and optical devices, it is necessary to have good quality single crystals in bulk form or in film form having a single crystal form (not a mixture of two or more polytypes) and having no or very few defects.
Previously known methods for producing a SiC single crystal include the sublimation method and the chemical vapor growth (CVD) method, which are methods of growth in vapor phase, and the solution growth method, which is a method of growth in a liquid phase.
In the sublimation method, a raw material which is SiC powder is sublimated at a high temperature of 2200-2500° C., and a SiC single crystal is recrystallized on a seed of a SiC single crystal disposed in a lower temperature zone.
In the CVD method, a silane gas and a hydrocarbon gas are used as raw materials, and a SiC single crystal is allowed to epitaxially grow on a substrate made of a Si or SiC single crystal.
In the solution growth method, carbon is dissolved in a melt of Si or a Si alloy to prepare a solution of SiC dissolved in the melt. A SiC seed crystal is immersed in the high temperature SiC solution in molten Si or a molten Si alloy as a solvent, and by making the SiC solution supercooled at least in the vicinity of the seed crystal, the SiC concentration in the solution is made supersaturated, thereby allowing a SiC single crystal to grow on the seed crystal.
In the solution growth method, there are two methods of creating a supercooled state, which are the temperature difference method and the slow cooling method. In the temperature difference method, a temperature gradient is established in the melt so that the temperature in the vicinity of the seed crystal is lower than in other portions of the melt, and the SiC concentration of the melt is supersaturated locally only in the vicinity of the seed crystal. In the slow cooling method, the entirety of a melt is gradually cooled, and the overall SiC concentration is made supersaturated. Since the slow cooling method is a batch operation, it is suitable for obtaining a SiC single crystal in thin film. In contrast, the temperature difference method allows a single crystal to continuously grow, and it is suited for the production of a SiC single crystal in bulk form.
It is easy for the sublimation method to produce a large bulk crystal, so commercial production of single crystalline SiC wafers is presently carried out by the sublimation method. However, a SiC single crystal which is grown by the sublimation method has problems with respect to the quality as a crystal in that it easily develops crystal defects such as hollow core defects referred to as micropipes, screw dislocations, and stacking faults.
The CVD method is used primarily for growth of SiC crystals in thin film due to its relatively slow growth rate. The quality of a thin film SiC single crystal is affected by the substrate. Due to the above-described problems with the quality of SiC substrates produced primarily by the sublimation method, there are limitations on improving the quality of thin films.
In the solution growth method which is a method of liquid phase growth, crystal growth occurs in a state close to thermal equilibrium, so it is possible to obtain single crystal having a markedly better crystallinity (free from contamination by different polytypes) than with vapor phase growth. As stated above, a melt of Si or a Si alloy is used as a solvent for a SiC solution.
However, various technical problems remain in obtaining SiC crystals of large area and high quality by the solution growth method.
Material Science and Engineering, B61-62 (1999) 29;-39, for example, describes that a technical problem in achieving high quality SiC is that when a SiC single crystal is grown from a solution having Si as a solvent, Si inclusions develop inside the SiC crystal. These inclusions are caused by a nonuniform surface in the growth interface referred to as a morphological instability. This nonuniform surface has a macrostep structure, and it is thought that Si which is the solvent enters between the macrosteps and is enclosed within a crystal by growth of the steps in the transverse direction.
J. Crystal Growth, 197 (1999) 147-154 describes that circular pits develop in the crystal surface in liquid phase epitaxial growth from a Si—Sc—C ternary solution. These pits are caused by foreign matter such as carbon particles which are deposited on the growth interface during crystal growth.
The term “inclusions” encompasses any phase which is incorporated into a single crystal from any cause and is different from the desired single crystal. When a SiC single crystal is grown by the solution growth method, a typical inclusion is a particle derived from droplets of Si from a solvent or C which are captured inside the crystal during crystal growth. This is caused by a nonuniform surface present at the crystal growth interface. When SiC crystals are grown by the solution growth method, the solubility of SiC in a melt of Si or a Si alloy is low, and the degree of supersaturation of the solution is often low, so step bunching easily occurs. As a result, the solvent is left between steps and becomes inclusions. Besides, silicides, carbides, nitrides, oxides, and the like can also become inclusions. Furthermore, a SiC crystal which is different from a desired polytype, such as a 3C—SiC crystal incorporated into a 6H—SiC single crystal, gas (bubbles) enclosed within a crystal, graphite particles incorporated into a solution, and similar substances can also become inclusions. A SiC single crystal having inclusions incorporated therein are unsuitable for use as devices.
FIG. 1 shows an optical photomicrograph of the cross section of a SiC single crystal containing inclusions which result from a Si solvent captured into a crystal and which were observed in growth experiments carried out by the present inventors. In the figure, the black portions at the ends of the downward pointing arrows are inclusions caused by enclosed solution.
In a single crystal, inclusions and pits are macro defects, which are not permissible in a material for devices. Such defects appear even during crystal growth on a research scale with a single crystal size of 2 inches or less. In addition, if the conditions of temperature or supply of a solute become nonuniform in a growth interface where crystal growth occurs, a nonuniform distribution in thickness develops in the resulting crystal. Particularly in a thin film single crystal, if the crystal thickness becomes nonuniform, the desired device performance can no longer be obtained, so such nonuniformity is also not permissible in a material for devices.
Technical challenges associated with high speed growth of SiC single crystals include increasing the growth rate while maintaining a high crystal quality. In general, in the growth of a SiC single crystal by the solution growth method, the growth rate is low. For example, in the case of solution growth using Si as a solvent, the growth rate at a melt temperature of 1650° C. is approximately 5-12 μm per hour. This growth rate is 1 to 2 orders of magnitude smaller than with the sublimation method. The reason why the growth rate of an SiC crystal is slow in the solution growth method is thought to be because the solubility of carbon in the solution and hence the SiC concentration are low.
The present inventors succeeded in increasing the solubility of carbon in a solution using a Si—Ti or Si—Mn solvent and growing a SiC bulk single crystal (JP 2004-2173 A1). However, even in that method, when the growth rate is increased, problems in the form of the above-described inclusions or nonuniformity of the growth rate inside the crystal plane may occur, and it is difficult to achieve stable high-speed growth of at least 100 μm per hour while maintaining crystal quality.
JP 2000-264790 A1 discloses a method in which a raw material comprising at least one transition metal element, Si, and C is heated to melt and the resulting melt is cooled to precipitate and grow a SiC single crystal. In this method, an optimal composition of solution is estimated based on a ternary phase diagram. However, in both experiments and calculations, there is almost no sufficiently is reliable Si—C-M system diagram (M=various elements) which is known at present, and it is not possible to determine an optimal composition of solution merely from a phase diagram. Even if an optimal composition of solution can be estimated, during cooling or under a temperature gradient for growth, various secondary products may be formed, and the seed crystal may dissolve or react with the crucible to cause damages to the crucible, so a SiC single crystal cannot always be stably obtained. Accordingly, it is necessary to determine an optimal composition by actually carrying out growth tests on various compositions using various solvents. The compositions which are specifically disclosed in JP 2000-264790-A1 are ones in which the transition metal element is Mo, Cr, or Co. In the case of Co, when expressing the molar concentration of Co as [Co] and the molar concentration of Si as [Si], it is reported that the value of [Co]/([Co]+[Si]) is preferably approximately 0.30.
As described above, in recent years, there has been a demand in industry to increase the size of SiC single crystals, but the larger crystals become, the more markedly appear defects such as inclusions, pits, and nonuniformity of the crystal thickness. Therefore, it has been thought difficult to stably produce a SiC single crystal with a size of at least one inch which does not have inclusions or pits and has a uniform growth thickness at a high growth rate of at least 100 μm per hour by the solution growth method.
In order to realize mass production of SiC single crystals, there is a need to determine an alloying element which can form a Si alloy having a high solubility for carbon in order to make high speed growth possible, which is available relatively readily, and which can be stably used.